1. Field of the Invention
This invention relates to an image forming method and an image forming apparatus such as a printer, a facsimile, a copier and the like, and more particularly to a method of and an apparatus for forming a visible image by developing an electrostatic latent image, formed on an photoconductor, an insulator member, a ferroelectric member or the like by electrophotography, electrostatic recording, ion flow method and the like, by causing liquid or solid charged particles to adhere to the electrostatic latent image.
2. Description of the Related Art
As a system for developing an electrostatic latent image formed on an electrostatic latent image support such as an electrophotography photoconductor, an electrostatic recording material, an insulator member, a ferroelectric member or the like, there have been known a dry development system using powder toner comprising coloring pigment included in solid fine particles and a wet development system using liquid toner comprising coloring pigment dispersed in insulating liquid. In the wet development system, since the coloring pigment is dispersed in liquid, the particle size of the toner can be small, which makes the wet developing system suitable for obtaining high resolution images. However use of combustible or volatile organic solvent has limited application of the wet development system, and recently the wet development system is hardly used.
In contrast, the dry development system is substantially larger in the particle size of the toner as compared with the liquid toner and the resolution and the quality of the image obtained are relatively low. However the dry development system is advantageous in that the use of dry toner facilitates handling the toner and miniaturization of the apparatus. Accordingly, the dry development system now prevails over the wet development system. Recently there are demands for image forming apparatuses which use the dry development system and yet are better in image quality and resolution. In order to meet such demands, it is necessary to further reduce the particle size of the toner, for instance, to not larger than 5 .mu.m.
In copiers and printers of electrophotography systems using liquid toner or solid toner, toner which has been applied to the electrostatic latent image support in development but does not contribute to forming an image is sometimes recovered and reused. (Such toner will be referred to as "non-used toner", hereinbelow.) However, since such non-toner constantly deteriorates in its characteristic due to reduction in density of the pigment and/or fluctuation in its toner/carrier ratio, only a part of the non-used toner has been reused. Further since a part of the toner is dispersed outside the apparatus, the non-used toner is not all recovered.
As another system for developing an electrostatic latent image on an electrostatic latent image support, there has been known a so-called mist development system in which atomized ink is dispersed in air like mist and is charged to form charged ink droplets or charged ink particles (referred to as "charged mist", hereinbelow), and then the charged mist is supplied to a developing space so that the charged mist adheres to the electrostatic latent image to develop it. Examples of the mist development system are disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 58(1983)-215671, 3(1991)-125171, 5(1993)-333703, and 6(1994)-95514, and "Electrophotography", vol. 16, No. 2, 1977, pp. 21 to 26. Further there have been known non-contact development systems where ink in the form of droplets is blown through a slit over the electrostatic latent image or, as disclosed in Japanese Unexamined Patent Publication No. 6 (1994)-295131, ink in the form of droplets held on a roller is blown onto the electrostatic latent image.
The charged mist can adhere to the non-image area, highlight area, or low-density area of the electrostatic latent image, or the latent image support outside the electrostatic latent image, which result in so-called fog. In order to prevent this, there has been known a system in which air free from the charged mist is flowed between the electrostatic latent image and the charged mist. Further there has been known a system in which an electrode biased to the same polarity as the charged mist is disposed to be opposed to the electrostatic latent image support with the charged mist intervening therebetween so that the charged mist is urged toward the electrostatic latent image and adhesion of the charged mist to the electrostatic latent image is promoted, thereby shortening the developing time.
The mist development system is advantageous over the conventional ink jet system which directly records an image on a printing paper in that improvement of image quality is facilitated since it uses finer ink droplets of the mist and that it requires no nozzle head which is required in the ink jet system and is formed by micro-machining technologies. Further by the mist development system, a development apparatus which can develop a wide area at one time and accordingly can form an image at high speed can be easily formed.
However it is generally difficult to supply the charged mist uniformly in the developing space, especially when the developing space is narrow, by supplying the charged mist to the developing space by use of an air fan or the like, and unevenness in density of the developed image due to fluctuation in concentration of the charged mist and/or unevenness in flow of the charged mist in the developing space is apt to occur. This is more serious when the distance between the electrostatic latent image support and the developing space and/or the cross-sectional dimensions of the flow passage of the charged mist in the developing space are not uniform, and greatly deteriorates the quality of the image. When the charged mist is supplied to the entire area of the developing space opposed to the electrostatic latent image support, charged particles can adhere to the non-image area, highlight area, or low-density area of the electrostatic latent image, or the latent image support outside the electrostatic latent image, which can result in fog.